The present invention relates to leak testing of storage tanks, particularly, an acoustic leak detection system using a small array of sensors for detecting leaks in storage tanks.
Leak testing of new storage tanks is now a routine matter. However, it is estimated that there are as many as 700,000 above ground storage tanks in the United States today. Their capacity ranges from 500 barrels to 500,000 barrels with the contents covering a wide range of petroleum products and chemicals. Operators and owners of large oil and chemical storage tanks require a sensitive test that may be applied routinely with minimal disruption to tank operations to locate storage tank leaks. The primary concern is with the environmental impact of a leak rather than a loss of product. Public concern, and pending regulations and the rising cost of clean up after a spill are focusing attention on leak testing. Regardless of the size of a tank, the potential for adverse environmental impact from storage tank leaks is greater than that from a tank or a pipeline failure. In case of a major spill, there is an immediate reaction and the bulk of the contaminants are cleaned up relatively quickly. A small leak is insidious. For example, in 5 years a 1/16 inch hole in a storage tank could leak half a million gallons or more of product, causing extensive contamination. Thus, the potential impact of this relatively small, if hard to find leak, is enormous.
Several methods for leak detection are known in the prior art. For example, soil testing monitors the external environment for the presence of leaked product. While this approach can detect very low levels of a specific substance, it cannot determine how or when the leak occurred. Also surface spills or other non-leak related releases may be mistaken as a leak.
Precision volumetric testing is effective on underground tanks, but its utilization is doubtful for above ground tanks and it does not give the leak location. The primary disadvantage of this method is the potential for significant changes in storage tanks and product volume for reasons other than leakage. For example, and not taking into account the effects of floor, shell or roof movement, a product temperature change of 0.01.degree. F. per hour will produce a volume change of 16 gallons per hour in a 55,900 barrel crude oil storage tank.
Inventory reconciliation is another method utilized today and is attractive as a continuous process but is limited by the measurement capacity of the instrumentation. This method also suffers from the same volume compensation problems as the precision testing method. Even if a leak is detected, its location is unknown. Leakage could be the result of a leak in a pump or piping, rather than the storage tank.
The tracer method of leak detection is very sensitive but it is also a very lengthy and costly procedure. Equipment setup and time for the tracer to percolate to a sample point could take days or even weeks. With careful use, the tracer method can detect very small leaks, although leak size and location cannot be determined.
The acoustic leak detection method is best suited for large atmospheric tanks. It relies on detecting acoustic signals that arise from leaks. Leak detection is limited by the level of signal from the leak compared to background noise. Determination of leak location is difficult because of interaction between differing leak and noise sources and the tank boundaries.
Heretofore, the conventional approach to acoustic leak detection utilized a ring of sensors spaced around the tank shell. This arrangement results in sensor spacings of twenty-five feet or more which presents difficulties with signal and noise discrimination. To locate an acoustic source utilizing the large array arrangement, the signal arrival times must be measured from three sensor positions or more. On detecting a signal at one sensor, the large array system must wait for the arrival of the signal at two other sensors. With sensors twenty-five feet apart, the delta-T window for the signal is of the order of five milliseconds and a reflection or signal from another source may be detected instead of the desired signal. A single leak can generate acoustic signals at a rate of 1,000 per second or 1 every millisecond, excluding reflections. Thus, in the large array arrangement one sensor could receive a signal directly from the leak, the second sensor could receive a reflection signal and the third sensor a noise signal. Under these circumstances, the timing of measurements are useless. In addition, the large array arrangement utilizes planar location for floor, roof and sidewall noise sources which are plotted in the same plane as leaks, thereby making interpretation difficult. This problem may be minimized by the use of guard sensors to lock out the array when upper level signals are detected. The disadvantage of guard sensors, however, is that the system cannot detect leak signals during the time it waits for noise signals to dissipate.
The large array acoustic leak detection arrangement utilizes standard acoustic emission systems adapted for leak detection. This results in an array set up and test procedure configured for the existing hardware rather than the specific application of detecting leaks in storage tanks. The present disclosure utilizes equipment developed solely for the purpose to locate leaks in storage tanks. The system of the present disclosure is a compact, digital system using novel sensor arrays designed specifically for use on storage tanks.
It is therefore an object of this invention to provide an acoustic leak detection system utilizing a small array of sensors placed inside or outside of a storage tank.
It is a further an object of this invention to employ a digital signal process to derive the relative arrival time of a signal at each sensor in the array.
It is yet another object of the invention to provide an acoustic leak detection system employing three dimensional source location to determine the location and nature of acoustic sources whereby incoming signals are screened for consistency in terms of velocity, arrival times, frequency and other attributes. This is accomplished by utilizing a small array arrangement of sensors whereby each sensor receives essentially the same signal at any given time. Multiple acoustic sources are thus distinguishable and unlikely to cause confusion as to the source.